The invention relates to an arrangement according to the preamble of claim 1 and the preamble of claim 6 for determining the remaining operating life of a fluorescent lamp.
Fluorescent lamp lighting fixtures are generally used owing to a long operating life and good color reproduction properties. The operating life of a fluorescent lamp is mainly determined according to the durability of cathodes, which, in turn, depends mainly on the number of fluorescent lamp ignitions. The fluorescent lamps used mainly in Europe are hot cathode tubes, where the cathodes are heated to a high temperature before the lamp is actually switched on.
The cathodes are built to resemble a resistance wire for heating the cathodes of the fluorescent lamps. The cathode surface comprises an active material providing ionization that is necessary for the operation of the lamp. Through a cathode resistor a filament current is conducted which heats the cathodes before the fluorescent lamp is switched on, thus facilitating the beginning of the ionization of the active material in the cathode. The cathodes are preheated by a ballast starter system, where the current flows through both cathodes and a starter during preheating. When the cathodes are heated enough, the starter stops conducting and disconnects the filament circuit. On account of the energy stored in the ballast during the heating of the cathodes, the current starts flowing in the fluorescent lamp and produces UV radiation. The UV radiation produced by a gas breakdown is absorbed into a phosphor layer on the surface of the lamp transforming the energy of the absorbed radiation into visible light.
The operating life of fluorescent lamps depends on the amount of active material on the cathode surface, and when the active material is used up, the fluorescent lamp stops functioning. The ionization on the cathode surface of the fluorescent lamp forms a hot spot at that particular point of the cathode where the ionization occurs and the current is transferred to the gas. The hot spot travels along the cathode as the lamp is used, and is on the new lamp close to the cathode terminal, which is connected to a higher potential. As the active material of the cathodes wears, the hot spot travels along the cathode surface.
Electronic ballasts are also used for igniting or burning fluorescent lamps. Deviating from the ballast starter arrangement, a filament voltage is always connected to the cathodes when an electronic ballast is used, and so a current is constantly flowing through the cathodes. Compared with the conventional solutions, electronic control gear provide advantages that include reduced losses and thus an improved light performance.
A problem with fluorescent lamps is to determine the time for changing the lamps. It is most economical to time the change in such a manner that as little as possible of the operating life of the fluorescent lamps is left unused. Very often fluorescent lamp lighting fixtures are difficult to put in place, which is why all fluorescent lamps located in one place should preferably be changed at the same time. A typical example of such a place is a factory hall, where the floor to ceiling height and the location of the lamps above the machines or equipment impede the change.
In vehicles, an anticipating signal indicating that fluorescent lamps are burnt out makes it easier to plan the service for a vehicle. The aim is to time the vehicle service so that as many as possible of the fluorescent lamps that are almost burnt out can be changed during the service. Selecting the same time for the vehicle service and for the lamp change may reduce the number of vehicle lay days. Examples of such vehicles to be serviced are buses, railway carriages or passenger ships.
It is previously known to anticipate the end of the operating life of a fluorescent lamp by measuring the lighting voltage between the cathodes in the lamp. Patent application EP 0 731 437 A2 presents an arrangement, by which a change can be detected in the lighting voltage, before the lamp stops functioning. According to the publication after detecting a change in the voltage the current supply is cut off, and the lamp slowly dims. A drawback with the equipment of the reference publication is that the voltage to be measured above the lamp is quite high, in which case the measurement equipment should also be constructed in accordance with corresponding voltage levels. The lighting voltage greatly depends on filling gas properties, operating temperature and current change when the power supply voltage varies. Due to the facts mentioned above determining the remaining operating life of a lamp based on measuring the lighting voltage between the cathodes is very unreliable.